Conventionally, a glass substrate is generally cut apart by so-called scribing, specifically by first securing the glass substrate on a workpiece stage of a cutting machine by vacuum chucking or the like, then forming a linear scratch called a scribed line, which actually is a kind of crack (hereinafter referred to as a crack also), on one surface of the glass substrate by the use of a wheel-shaped cutter having the outermost edge thereof formed of a superhard material, such as superhard alloy or diamond, and then pressing the glass substrate from behind the opposite surface thereof along the scribed line by the use of a pressing means, such as a press or roller, so that the crack that has been formed in the scribed line on the glass substrate in the direction perpendicular to the substrate surface develops until eventually the glass substrate breaks apart.
Today, liquid crystal display elements and other display devices similar thereto are fabricated, with a view to fabricating them at low costs, by cutting apart a large-format glass substrate material, having thin films, such as a transparent electrode like an ITO film, an insulating film, an orientation film, and the like formed on the surface thereof beforehand, into a shape having desired dimensions. When such a glass substrate having deposited-film layers (hereinafter referred to as deposited films also) formed thereon is cut apart, various problems arise that were unimaginable in the scribing of a conventional glass substrate. For example, scribing the film-deposited surface results in destroying the deposited films around the scribed lines. This makes the deposited films scatter in the form of fine particles, and may lead to unacceptable product quality. Even scribing the opposite, non-film-deposited surface results in the deposited films being scratched or deformed as a result of the film-deposited surface making contact with the stage of the cutting machine.
A solution to these inconveniences is proposed in Japanese Patent Application Laid-Open No. H11-64834, which discloses a conventional method for scribing a large-format glass substrate having deposited films formed thereon into strip-shaped sections having desired dimensions. This method attempts to overcome the above inconveniences by scribing the glass surface opposite to the film-deposited surface.
Now, with reference to FIG. 52, the method disclosed in Japanese Patent Application Laid-Open No. H11-64834 will be described, mainly by quoting from its specification. A glass substrate 101 has a deposited film 102 formed on one surface thereof. The glass substrate 101 is secured, with the deposited film 102 up, on the top surface of a surface plate 103 serving as a workpiece stage by an unillustrated chucking means, such as vacuum. In the surface plate 103, rectilinear openings 104 are formed at predetermined intervals so as to permit a scribing means 105 serving as a crack-forming means for forming a scribed line, i.e., a crack, to move along the openings 104 while scribing the bottom surface of the glass substrate 101. A pushing means 106 and a positioning means 107, each realized with an air cylinder or the like, work together to move the glass substrate 101 into one predetermined position after another. The pushing means 106 has a pushing pin 110 provided at the tip thereof, and the positioning means 107 has a positioning pin 111 provided at the tip thereof. A pressing means 109 presses the glass substrate 101, from above the film-deposited surface thereof, against the top surface of the surface plate 103 so as to hold the glass substrate 101 in position.
Next, how the method works will be described. When the surface plate 103 is located in position “a” indicated with dotted lines, the glass substrate 101 is placed thereon by a unillustrated transporting machine, and then the positioning pin 111 at the tip of the positioning means 107 swoops down on the surface of the surface plate 103 in response to a positioning signal from a unillustrated controlling means. Next, the pushing pin 110 at the tip of the pushing means 106 extends so as to move the glass substrate 101 on the surface plate 103 in the direction indicated by a horizontal arrow in the figure until the glass substrate 101 makes contact with the positioning pin 111. In this way, the places at which to cut apart the glass substrate 101 are aligned with the openings 104. Next, the controlling means gives instructions to chuck the glass substrate 101 onto the surface plate 103, then move the pressing means 109 to the place where scribing is going to be performed and press it against the top surface of the glass substrate 101, and then make the scribing means 105 scribe the bottom surface of the glass substrate 101. The scribing means 105 repeats scribing while traveling from one opening to another by being fed at the intervals at which to form scribed lines. Thus, according to this method, the glass substrate 101 is scribed from below the bottom surface thereof through the openings 104 formed in the surface plate 103. Here, the force with which the glass substrate 101 is chucked onto the surface plate 103 by the chucking means is not sufficient to counter the scribing load exerted by the scribing means 105, and this insufficiency needs to be compensated for by pressing the glass substrate 101 from above the film-deposited surface thereof with the pressing means 109.
The conventional method and apparatus for scribing (cutting-apart) described above are intended mainly for the cutting-apart of a glass substrate having a protective film formed on a large-format glass material. With a glass substrate having a thin film, such as an ITO film, formed thereon, however, the use of a common pressing means, such as the weight-shaped one 109 shown in FIG. 52, for countering the scribing load from below results in the thin film being destroyed by the load of the pressing means itself.
For further cost reduction of liquid crystal display elements, attempts have been made to cut apart a large-format glass material after forming films, such as a polarizer plate and a protective sheet, thereon instead of bonding those films after the cutting-apart of the glass substrate as conventionally practiced. It has been customary to bond polarizer plates to the outer surfaces of an upper and a lower glass substrate in the last step of the process of fabricating a liquid crystal cell. This, involving positioning of the films relative to the glass substrates and requiring an extra step, hampers cost reduction
The thickness of a deposited film varies according to its type; while a thin film, such as an ITO film, is a few μm thick or thinner, a film layer, such as a polarizer plate, is 10 μm to 0.6 mm thick. With a glass substrate having such a film layer formed thereon, it is impossible to directly scribe the film-deposited surface thereof. With the conventional method and apparatus for scribing described above, the presence of the film layer makes it impossible to cut apart the substrate after scribing.
On the other hand, in the case of medium- to small-size liquid crystal panels, in particular those with screen sizes up to about 5 inches, it has been customary to fabricate them by first cutting apart large-format glass substrates already bonded together roughly into strip-shaped glass substrates, then processing them in predetermined manners, as by putting and sealing liquid crystal in between, then further cutting them apart finely into a predetermined panel size to produce a plurality of individual cells, and then bonding polarizer plates to the individual cells to produce a plurality of liquid crystal panels.
Now, with reference to FIG. 53, the liquid crystal panel fabricated by this conventional method will be described. In FIG. 53, there are shown perspective external views of the conventional liquid crystal panel, with the figure at (a) showing the top side thereof and the figure at (b) showing the bottom side thereof. This liquid crystal panel 550 is composed of a pair of substrate cells 551a and 551b bonded together and having liquid crystal sealed in between. One end of one 551a of the substrate cells protrudes from one end of the other 551b, and, on the inner surface of the protruding portion 551aa are formed connection terminals 553 by way of which the liquid crystal panel is driven. Moreover, on the outer surfaces of the substrate cells 551a and 551b, polarizer plates 552a and 552b are respectively bonded so as to cover the display region (not shown). When the liquid crystal panel is designed for use in a so-called backlit liquid crystal display device that achieves display by transmitting light from a light source, the polarizer plates 552a and 552b have roughly equal sizes and are so arranged as to face each other with the liquid crystal cells 551a and 551b sandwiched in between.
The conventional method described above involves bonding polarizer plates one by one to individual cells, resulting in extremely poor fabrication efficiency. Even when a dedicated machine is used for that purpose, the influence of static electricity imposes a limit on the rate at which polarizer plates can be bonded (typically, 8 to 10 seconds required per polarizer plate). Thus, to cope with demands for high yields (fabrication of as many liquid crystal panels as possible) on the market, a large number of polarizer plates need to be treated concurrently on a large number of machines. This greatly increases plant-and-equipment spending, and thus increases the costs of liquid crystal panels as end products.
A way to avoid this is proposed, for example, in Japanese Patent Application Laid-Open No. H6-342139, which discloses a method for fabricating a liquid crystal panel whereby first a polarizer plate having cut lines, marking where to cut apart, formed at predetermined places thereon is bonded to a plastic substrate, and then the plastic substrate is cut apart along the cut lines to produce a plurality of liquid crystal panels. According to this method, the polarizer plate is bonded to the substrate before the substrate is cut apart. This helps greatly scale down the step itself of bonding the polarizer plate. Thus, it is possible to improve fabrication efficiency without unduly increasing plant-and-equipment spending.
This method, however, has the following disadvantages. First, a polarizer plate itself is formed of polyvinyl alcohol sandwiched between layers of cellulose triacetate or coated with an acrylic resin, and is formed as a thin film about 0.2 to 0.6 mm thick. Therefore, when cut lines are formed in this polarizer plate, unexpected application of a load thereto may deform the portion thereof around the cut lines, eventually leading to warping or breakage of the polarizer plate. In particular, when bonded to a substrate, the polarizer plate needs to be bonded thereto so that the cut lines are located at predetermined places on the substrate. This necessitates the use of a high-precision machine, which is disadvantageous to cost reduction of liquid crystal panels.
Second, when the substrate is cut apart, the polarizer plate is also cut apart (even where a cut line is formed, the portions of the polarizer plate across it are separated from each other). In particular when a glass substrate, which is brittle, is used as the substrate, the substrate and the polarizer plate have quite different properties, and therefore, unless special care is taken, the glass substrate may break at inappropriate places, or the polarizer plate may exfoliate unexpectedly. That is, it is extremely difficult to cut them apart without degradation in quality. Thus, this method leaves room for improvement. Incidentally, Japanese Patent Application Laid-Open No. H6-342139 presupposes the use of a plastic substrate, of which the material is similar to the material of the polarizer, as the substrate, and therefore, quite naturally, it gives no special consideration to the method for cutting them.
On the other hand, in the conventional liquid crystal panel shown in FIG. 53, the individual substrate cells 551a and 551b are thin (when formed of glass, about 0.4 to 0.7 mm), and therefore the protruding portion 551aa, in particular, is mechanically weak. Thus, when the liquid crystal panel is transported from one place to another or assembled into a liquid crystal display device, if it is hit or dropped, the protruding portion 551aa may be cracked or deformed, or broken at a corner. Accordingly, extremely cautions handling is required.
Moreover, in recent years, a polarizer plate itself has come to be given multiple functions, and is formed of a plurality of sheets having various optical properties laid on one another. As a result, at the edges of a polarizer plate bonded to a substrate cell, there are often observed burrs and the like. This makes it difficult to assemble a liquid crystal panel into a liquid crystal display device, or causes the polarizer plate to exfoliate from the substrate cell unexpectedly. Furthermore, in a case where a removable protective film is laid on the outer surface of a polarizer plate so as to be integral therewith, when the liquid crystal panel is transported from one place to another or assembled into a liquid crystal display device, the film may exfoliate from the polarizer plate unexpectedly and scratch the polarizer plate itself.